1. Field of the Invention
The present invention relates to a method for driving an image displaying apparatus using a subfield method to be applied in transmission type or projection type display or viewfinder, liquid crystal image apparatus such as head-mounting type display, plasma image displaying apparatus, digital mirror image displaying apparatus, electroluminescence image displaying apparatus, field emission image displaying apparatus, etc.
2. Description of the Related Art
In general, in a displaying apparatus based on binary display such as a plasma display or a display using digital mirror device, a subfield method is used to obtain intermediate gradation. According to this method, one field is divided into a plurality of subfields with weighting of luminance of light emission according to a predetermined law, i.e. by dividing one field into a plurality of subfields with different relative ratios of luminance, and intermediate gradation is displayed by superimposing the gradations over one field.
In recent years, displaying is performed by the subfield method on the displaying apparatus such as liquid crystal displaying apparatus or electroluminescence displaying apparatus, which can provide analog gradation. This is from the reason that, in analog gradation, variation and noise of image cell are very likely to exert influence, while, in the driving using the subfield method (hereinafter referred as “subfield driving”), the influence from variation and noise of image cell hardly occurs. When subfield driving is performed on nematic liquid crystal, it is particularly effective because the subfields to be divided in one field can be increased. Also, in the subfield driving, image signals are digitized and these can be expressed in binary values. This is more advantageous in terms of cost than analog conversion, and it is also suitable for mass production.
The image displaying apparatus of these types differ in structure and driving mode, but problems arise in that pseudo-contour occurs when animated picture is displayed due to the subfield driving as described above. For example, it is supposed here that one field has a plurality of 8-bit subfields, and these subfields have weighting of 1:2:4:8:16:32:64:128. By combining these subfields, expression can be given on gradations of 0 to 255, i.e. on 256 gradations. In this case, it appears that the pseudo-contour in animated picture is caused by time gap in light emission timing when the subfields are displayed. When moving speed of an image is high, time gap is converted to spatial lag. As a result, animated picture pseudo-contour occurs and this results in deterioration of image quality of animated picture.
FIG. 20 is a drawing to schematically explain generation of animated picture pseudo-contour. In FIG. 20, there are subfields of 1 to 8 (hereinafter, the subfields 1 to 8 are referred as “SF1 to SF8”). In the pixels adjacent to each other, gradation level shows a boundary of “127” with “128”. In FIG. 20, rightward direction represents the flow of time, and SF1 to SF8 are displayed sequentially in this order. An open portion (white portion) represents off-display (in black), and the shaded portion shows on-display (in white). The higher the gradation level is, the closer it is to white color. The lower the gradation level is, the closer it is to black color. S1 to S3 as shown by arrows in FIG. 20 indicate positional shifting of visual line in the direction of height. In case the visual line is fixed at upper pixel position as shown by the arrow S1, a light passing through the white portion (indicating pixels of on-display) comes into the eyes of an observer in SF1 to SF7. In the subfield SF8, a light passing through the black portion (indicating pixels of off-display) comes into eyes. As a result, gradation level can be adequately recognized.
However, when visual line is shifted to the upper pixel position from the lower pixel position as shown by the arrow S2, a light passing through the black portion in all of SF1 to SF8 comes into eyes, and gradation level is turned to “0” (entirely in black). Therefore, due to the shifting of visual line, a line of black level appears falsely, and this is seen as a pseudo-contour.
On the other hand, when visual line is shifted from the upper pixel position to the lower pixel position as shown by the arrow S3, a light passing through white position comes into eyes of an observer in all of the subfields SF1 to SF8, and the gradation level is turned to “255” (entirely in white). Therefore, due to the shifting of visual line, a line of white level is generated falsely, and this is seen as a pseudo-contour.
Also, even when the image is not moving, display timing is different at the boundary between the pixels of “127” and the pixels of “128”. As a result, a phenomenon such as pseudo-contour disturbance may occur, in which the boundary becomes conspicuous when the observer blinks.
As one of the methods to suppress the generation of the animated picture pseudo-contour as described above, a subfield pattern as shown in FIG. 21 is proposed. This is a method to divide a subfield with long display time among the subfields into a plurality of subfields with short display time (e.g. see the patent reference 1 as given below). In this example, SF7 and SF8 with total weighting of 192 are divided into 6 subfields with weighting of 32 each. In case the boundary between gradation levels “127” and “128” is shifted, the gradation level is not turned to “0” or “255” due to the shifting of visual line shown by the arrows S1 to S3 as already explained in connection with FIG. 20, and this reduces the occurrence of pseudo-contour.
Also, when vertical synchronizing signal frequency is low, improvement is made to large area flicker disturbance, which occurs in case of high luminance display (e.g. see the patent reference 2 as given below). In FIG. 22, a subfield with weighting 32 is further divided into two subfields, and time interval between the two subfields is turned to about one-half of the field, i.e. it is the so-called two-crest arrangement.
In these methods, solution is given so that extreme change does not occur in the selection pattern of subfields due to the change of the gradation level.
[Patent Reference 1]
JP-A-8-254965 Publication (Paragraph 0014-Paragraph 0015; FIG. 1)
[Patent Reference 2]
JP-A-2001-42818 Publication (Paragraph 0023; FIG. 8)
It seems that the animated picture pseudo-contour, the pseudo-contour disturbance, or flicker disturbance as described above are caused by the time gap in the weighting of light emission pattern within the field. Specifically, in all of these methods, temporal weighting of the subfield selected by gradation is different in each case.